Seismic surveys, when used in conjunction with other available geophysical, borehole, and geological data, provide information about the structure and distribution of subsurface rock types and their interstitial fluids. Oil companies rely on interpretation of such seismic data for selecting the sites in which to invest in drilling exploratory oil wells. Even though seismic surveys provide maps of geological structures rather than direct measurements of petroleum, seismic surveys have become a vital part of selecting the site of an exploratory and development well. Experience has shown that using seismic surveys greatly improves the likelihood of a successful venture.
When borehole logs are available from nearby wells, seismic survey data can be enhanced by combining it with the log data. Various methods exist for combining the different data types, but at least some of the methods involve the use of a synthetic survey.
To obtain a synthetic survey, analysts first employ the borehole logs to make determinations of density, porosity, fluid type, shear wave velocity and pressure wave velocity in each of the formation layers. The analysts then construct a mathematical model of the formation and simulate its response to a seismic survey. Just as in a real survey, in the simulation a seismic wave propagates through the model and reflects where the acoustic impedance changes. Energy from the reflected waves travels to receiver positions where the energy is recorded in the form of a seismic trace.
The traces from a synthetic survey perfectly scaled and representative of reflection signal strengths. However, real world seismic surveys require the use of physical transducers, which introduce scaling errors. Scaling errors cause inaccuracies in seismic attributes calculated from seismic surveys. Synthetic surveys provide a means for identifying and correcting at least some of these scaling errors.
At least some existing methods provide for normalization of seismic traces relative to synthetic traces, but to date these methods fail to adequately account for the dependence of scaling error on incidence angle. Such a dependence is common, and a method that provides for compensation of such an error dependence would be expected to significantly improve the accuracy of seismic attribute calculations, thereby further enhancing the likelihood of success in discovering and exploiting hydrocarbons, ores, water, and geothermal reservoirs.